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How well do you need to know it?

What factors should you look for in flowmeter selection relating to the output or display?

Precision. Often misunderstood, but in the most part, it’s what matters in measurement. It’s the degree to which repeated measurements under unchanged conditions show the same results.

Accuracy. The degree of closeness of measurements of a quantity to that quantity’s actual (true) value.

Linearity. For flowmeters it’s the curve of accuracy compared against flow rate.

Resolution. If the digital display only has 3 digits then selection of the units has more effect than the accuracy (etc.) of the meter itself. For example, set up with a maximum of 110 US gallons/min the resolution of ±1 US gallon per minute is 1% at best and 5 or 10% or worse at lower flows. Changing over to litres improves the resolution by a factor of 4. More importantly this shows the value of having enough display digits to match the users requirements and, probably, the accuracy of the meter.

Traceability. So the supplier gives you a set of data, a claim of performance. All meaningless without reference to something solid, something comparable like a National Standard.

ISO 5725. According to ISO 5725-1, the terms trueness and precision are used to describe the accuracy of a measurement. Trueness refers to the closeness of the mean of the measurement results to the actual (true) value and precision refers to the closeness of agreement within individual results. Therefore, according to the ISO standard, the term “accuracy” refers to both trueness and precision.

ISO17025. Simply a laboratory standard: General requirements for the competence of testing and calibration laboratories.

Repeatability. Another word meaning precision but often taken as the closeness of one set of results to some more with conditions unchanged. Probably should include some reference to time and:

Hysteresis. In some systems the precision varies according to whether the flow is increasing to the measurement point or decreasing. In particular near the start-up flow rate it may be found that, with the flow increasing from zero, the meter provides an output at ‘x’ whilst, when the flow decreases the meter may continue to provide an output at lower than ‘x’.

Long term accuracy. This could be restated as: will it measure the same tomorrow as it does today and what about next year?

Recalibration. The periodicity at which the meter should be recalibrated is not set in stone. Some meter types are less stable than others. Where the meter is used to calculate tax or fiscal amounts then a daily recalibration is sometimes necessary. In a benign fluid, with flow rates kept within bounds then others might need checking every 10 years. Litre Meter recommend a yearly check at first, analysis of the results, then an increased period depending on the customers needs.

On site calibration. Whilst every flowmeter that Litre Meter manufactures is calibrated in laboratory conditions on a similar fluid and at a steady flow rate there are differences such as meter orientation and pressure pulsation. Whilst pressure pulsation won’t affect positive displacement meters it can have a severe effect on turbines, for example. So Litre Meter recommend that each meter is calibrated in-situ. Various techniques are described in the flowmeter manual.

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[…] How well do you need to know it? In other words, to what accuracy and repeatability do you need the result? And, if that isn’t the same over the whole range of the meter or your application where would you prefer the accuracy to be? Oh, and don’t forget, accuracy costs – are you prepared to pay for it? […]